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In a river, water flows at speed of $1\mathrm{mps}$. A boat, which moves at speed of $4\mathrm{mps}$ in a calm water, carries mail to a little town distant $6\mathrm{km}$ downstream. How long will it take till the boat returns? (We disregard the time necessary for mail-handover.) Note: Abbreviation "mps" stands for meter per second.